Method and apparatus for preventing coke formation in distillation towers



Jan. 27, 1953 J. s. PALMER METHOD AND APPARATUS FOR PREVENTING coxa FORMATION IN DISTILLATION TOWERS Filed Au ..25, 1948 FIG. 5.

FIG. 3.

suPnneAno vnon Patented Jan. 27, 1953 METHOD AND APPARATUS FOR PREVENT- ING COKE FORMATION IN DISTILLATION TOWERS Jewell S. Palmer, Baytown, Tom, assignor, by

mesne assignments, to Standard Oil Development Company, Elizabeth, N. J., a corporation of Delaware Application August 25, 194:8, Serial N 0. 46,156

4 Claims. 1

The present invention is directed to a method and apparatus for preventing coke formation in distillation towers. More particularly, the invention is directed to a method and apparatus including a distillation tower in which a stream of superheated hydrocarbons are injected into a zone maintained at a lower temperature than the superheated hydrocarbons.

Prior to the present invention, it has been known to distill hydrocarbons at high temperatures. It has also been known to prevent formation of coke in high temperature equipment by suddenly reducing the temperature of the hydrocarbons below the temperature at which reaction occurs. It has been observed that coke formation will not occur if hydrocarbons are above their dew point. On the other hand, if hydrocarbons are at their dew point and are maintained at a relatively high temperature, the

hydrocarbons tend to form coke quite readily.

The industry, therefore, naturally tries to operate equipment in which coke tends to form either above or below the dew point. For example, in high temperature conversion operations, it is necessary to reduce the temperatures of the converted hydrocarbons rapidly through the temperature region where coking occurs. Stating this otherwise, it is necessary to reduce the temperature of hydrocarbons past their dew point so that coke formation will not occur. Many expedients have been adopted for reducing rapidly the temperatures of hydrocarbons. Such expedients include quenching the hydrocarbons with cooler hydrocarbons or with water or by indirect heat exchange with cooler material.

However, in distillation towers and the like localized areas may obtain where the hydrocarbons in a superheated condition may be cooled to their dew point and thus be caused to form coke. tillation tower to which is charged hydrocarbon vapors in a superheated condition. Thus, for example, in a distillation tower forming an auxiliary part of a catalytic cracking operation, the

converted products in a superheated, vaporous 4.

condition are charged into the lower portion of a distillation tower. As long as the vapors are in a superheated condition, there is no difficulty of coke formation in the conduit forming the inlet to the distillation tower. virtue of the fact that there is radiation from the walls of the hot inlet line and from the vapors charged to the tower to the relatively cool interior walls of the distillation tower and to the cooler mass of oil in the tower, the vapors i Such a situation may obtain in a dis- However, by 5;

all

(Cl. l.96--139) lose heat rapidly and a localized zone is caused to exist in the conduit at the point where the vapors enter the tower at which point the vapors may be at their dew point. These vapors are subjected to relatively high temperature even though they may be at their dew point and, consequently, tend to form coke very readily. This coke formation has been observed to occur in the form of a coke stalagmite. This stalagmite of coke ultimately builds up until it causes an appreciable pressure drop in the inlet conduit leading into the tower. Ultimately the pressure drop becomes sufficiently great to cause the coke to break loose from the interior surface of the inlet line and then flow into the tower. The cycle is then repeated until coke builds up to a point or accumulates in the bottom of the distillation tower such that the tower may become inoperable and must be removed from the operating stream for cleaning out purposes.

It is the main object of the present invention to provide a method and apparatus whereby distillation towers to which are charged superheated vapors are maintained substantially free of coke which heretofore tended to form in the inlet line thereto.

Another object of the present invention is to provide a method and apparatus whereby superheated vapors charged to a distillation tower are maintained at a temperature substantially above their dew point.

The objects of the present invention may be achieved by providing an inlet line to a distillation tower with a restriction at the point where the inlet line enters the tower. The restriction may be in the form of a so-called artificial obstruction welded to the upper interior surface of the inlet line or it may be provided by constricting the inlet line to reduce the area of flow to approximately half the area of flow of the inlet line. By providing a restriction in the inlet line, the velocity of the superheated vapors is increased and also by maintaining the temperature of the heated vapors substantially above the dew point thereof, coke formation is substantially prevented. On the other hand, if the dew point should be reached as the vapors enter the tower, the restriction in the inlet line causes the droplets of liquid to be swept off immediately before coke can form therefrom.

Briefly, the present invention may be described as embodying a distillation tower, one inlet line by way of which superheated vapors are introduced into the lower portion of the tower. An outlet line should be located in the upper portion of the tower above the midpoint thereof while the inlet line should be located below the midpoint of the tower and preferably adjacent the bottom of the tower. The inlet line at its point of juncture with the tower is provided with a restriction which may be an interior obstruction defining an area of flow in the inlet line of at least one-fourth and not greater than three-fourths of the area of flow of the inlet line. Preferably the restriction should define an area of flow of about one-half the area of the inlet line. This restriction may define a space with an inner and outer surface which may be filled with an insulation material resistant to high temperatures. Such insulation material may include magnesia, asbestos fiber, glass fiber, and many of the numerous insulation materials well known to the art. The purpose of the insulation material is to prevent loss of heat from the outer surface of the restriction means and thus allow maintenance of the temperature of the superheated vapors at substantially above the dew point thereof.

The invention will be better understood by reference to the drawing in which Figure 1 is an elevation partly in section of an embodiment of my invention showing its 'plication to a distillation tower;

Figure 2 is similar to the embodiment of Figure 1 but shows the lower portion of the distiila- 9 tion tower with a detail of the restriction means;

Figure 3 is a sectional view of the embodiment of Figure 2 taken along line IIIIII of Figure 2;

Figure 4 is another and a preferred embodiment of my invention showing a detail of a restriction means and inlet line in the lower part of a distillation tower; and

Figure 5 is a sectional view of the inlet line with the restriction therein taken along line V--V of Figure 4.

In the several figures of the drawing, identical numerals will be employed to designate identical parts.

Referring now to the drawing and particularly to Figure 1, numeral II designates a distillation tower provided with trays I2 on which may be located bell caps and other vapor-liquid contacting means. The vapor-liquid contacting means are conventional to the art and for purposes of simplifying the description will not be described here. Distillation tower I I is provided with an inlet line I4 and at least one outlet line I5 located in the upper part thereof. Inlet line Ed is shown in the lower portion of the distillation tower and should be below the midpoint thereof and preferably adjacent the bottom. Line It describes substantially a right angle with the longitudinal axis of tower II. The distillation tower may also be provided with side stream drawoff lines IE, IT, and I8, which may be put into operation by manipulation, respectively, of valves I9, 20 and 2|. Tower II is also provided with a bottoms outlet line 22 for withdrawal of heavy fractions therefrom.

Inlet line I4 defines a passageway through which superheated vapors are introduced into the tower I I. Located in inlet line It is a restriction 23 which may reduce the area of flow of line I4 by approximately one-half and cause the temperature of the superheated vapor flowing through line I4 into tower II to be maintained substantially above its dew point.

In operating the apparatus of the present invention, superheated vapors are introduced into the tower II by line: [4 and on passage 4 through the restricted opening 24 defined by restriction 23 prevents formation of coke in the inlet line I4. By providing a space in the interior of restriction means 23 and filling this space with insulation material of the type illustrated, it is possible to maintain the temperature at this point substantially above the dew point. Without such a restriction, the superheated vapors on entry into the tower II through inlet I4 lose heat rapidly to the cooler body of oil therein and to the cooler walls of tower II and thus allows the vapors to reach their dew point and cause formation of coke. From the foregoing description taken with the drawing it will be apparent that the tower II has an area of fiow substantially greater than the area of flow in line I4. Further, it will be apparent that the area of flow in tower II increases abruptly at the point where line I4 joins the tower I I. Thus the flow of the restricted superheated stream is directly from the line M into the tower I I.

Referring now to Figure 2, it will be seen that a detail of the restriction means 23 is given. Restriction means 23 is shown in section and is located at the point where the inlet line I5 joins tower II. The space 25 may be filled with insulation material shown as the shaded portion 25. It will be desirable also to insulate the inlet line I4 with similar insulation material 21. The cross sectional view of Figure 2 is shown in Figure 3.

Referring now to Figure 4, a preferred embodiment in section of a detail of a distillation tower showing another modification thereof is given. In this particular embodiment, the conduit It includes a constricted portion 33 which defines an area of fiow at its juncture with tower I i approximately half the area of flow of conduit i l. Thus, the conduit I4 has its size suddenly reduced by the constriction 35 which forms a constricted passageway 3| leading into the tower which defines an area of flow approximately one half the area of flow of conduit I4 where it enters the tower at 32. Conduit I4 and reduced or constricted passageway 3| may be suitably insulated with insulation means 33 of the type illustrated. The constricted portion 35 defines a space 3-; with its outer surface and with the outer surface of tower it. This space may also be filled conveniently with insulation material to prevent loss of heat from the superheated vapors flowing through the space defined by conduit i4 and restricted passageway 3|.

It will be seen in the embodiment of the invention described in Figures 4 and 5 that the super heated vapors have their velocity suddenly increased as they flow from inlet i l through passageway 3i and into the tower i I through the area of flow 32. By virtue of the fact that the space 35 is insulated and the velocity of flow through the passageway 3! is increased, the temperature of the interior surface of the inlet -d of the vapors passing therethrough is tained substantially above the dew point therein.

Generally speaking, the extent to which the cross sectional area of line It is restricted at the point of juncture with distillation tower II will depend on the flow rate of superheated vapors in the line, the temperature of these Vapors, and the amount of radiation from the walls of the inlet line to the inner walls of the distillation tower. For most purposes a restriction of about 50% of the area of the inlet line has been found to be satisfactory. If the restricted area is more than about 75% of the original area of the inlet line, the velocity of flow will generally not be sufliciently high at the point of entrance into the tower to prevent droplet accumulation at the point of juncture. On the other hand, it is generally undesirable to restrict the area to the extent of less than about of the cross sectional area of the inlet line since the velocity of the superheated vapors is increased to such an extent as to cause undesirably high pressure drops in the system.

While the method and apparatus of the present invention has application to any distillation tower to which superheated vapors are charged, it is particularly applicable to distillation towers forming an auxiliary part of catalytic cracking units, and particularly a fluid catalytic cracking unit. In such catalytic crackin operations, converted products in a vaporous condition at a temperature in the range from about 880 to 980 F. are introduced by way of an inlet line into the lower portion of a tower similar to tower II. The oil in the bottom of the tower is at a temperature in the range from about 650 to 720 F. and consequently causes the superheated vapor to lose heat rapidly to the body of oil in the tower. As the heat is lost from the superheated vapors from their point of entry, these vapors rapidly reach their dew point and cause the formation of coke at the critical point of entry into the tower. It will thus be seen that the various embodiments of the present invention prevents the formation of coke by maintaining the vapors at substantially above their dew point until they reach the interior of the tower,

In lieu of insulating the conduit [4 defining the inlet to the tower, it may be desirable to provide an auxiliary source of heat therefor. Such source of heat may include a jacket through which a heating means may be flowed adjacent the tower at the point of entry of the inlet line or it may include a resistance coil connected by suitable leads to a source of electrical energy to provide heat for the point where vapors enter the tower. Preferably, however, the critical point where coke formation has been observed is insulated against loss of heat by radiation.

The nature and objects of the present invention having been fully described and illustrated, what I wish to claim as new and useful and to secure by Letters Patent is:

1. Apparatus for distilling hydrocarbons which comprises, in combination, a distillation tower provided with internal vapor-liquid contacting means and means for inducing reflux therein, a first conduit dischargin into the lower portion of the tower as an inlet line defining substantially a right angle with the longitudinal axis of said tower, a second conduit discharging from the upper portion of the tower as an outlet line, a third conduit discharging from the lower portion of the tower below the inlet line, and a permanently fixed restriction means in the upper portion of the first conduit at the point it enters said tower defining an area of flow no more than and no less than the area of flow of the remainder of said first conduit, said restriction means defining an inner and outer surface with the outer surface cooperating with the interior surface of said first conduit to define a space, and insulation means filling said space.

2. Apparatus for distilling hydrocarbons which comprises, in combination, a distillation tower provided with internal vapor-liquid contacting means and means for inducing reflux therein, a first conduit discharging into the lower portion of said tower as an inlet line defining substantially a right angle with the longitudinal axis of said tower, and a second conduit discharging from the upper portion of said conduit as an outlet line, a third conduit discharging from the lower portion of said tower below said inlet line, said first conduit defining a permanently fixed restriction having an area of flow at the point it enters the tower in the range of 25% to of the area of flow of the remainder of said conduit and defining a recessed space with its upper outer surface and the exterior surface of said tower at its point of juncture.

3. Apparatus in accordance with claim 2 in which said recessed space is filled with a body of insulation material.

4. Apparatus for distilling hydrocarbons which comprises, in combination, a distillation tower provided with internal vapor-liquid contacting means and means for inducing reflux therein, a first conduit discharging into the lower portion of the tower as an inlet line defining substantially a right angle with the longitudinal axis of said tower, a second conduit discharging from the upper portion of the tower as an outlet line,

a third conduit discharging from the lower portion of the tower below the inlet line, the inner surface of said first conduit defining a permanently fixed restriction having an area of flow at the point where it enters the tower which is in the range of 25% to 75% of the area of flow of the remainder of said first conduit.

JEWELL S. PALMER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS page only), Pub. by Gulf Publishing Co., Houston, Texas. 3d edition (1930). 

1. APPARATUS FOR DISTILLING HYDROCARBONS WHICH COMPRISES, IN COMBINATION, A DISTILLATION TOWER PROVIDED WITH INTERNAL VAPOR-LIQUID CONTACTING MEANS AND MEANS FOR INDUCING REFLUX THEREIN, A FIRST CONDUIT DISCHARGING INTO THE LOWER PORTION OF THE TOWER AS AN INLET LINE DEFINING SUBSTANTIALLY A RIGHT ANGLE WITH THE LONGITUDINAL AXIS OF SAID TOWER, A SECOND CONDUIT DISCHARGING FROM THE UPPER PORTION OF THE TOWER AS AN OUTLET LINE, A THIRD CONDUIT DISCHARGING FROM THE LOWER PORTION OF THE TOWER BELOW THE INLET LINE, AND A PERMANENTLY FIXED RESTRICTION MEANS IN THE UPPER PORTION OF THE FIRST CONDUIT AT THE POINT IT ENTERS. SAID TOWER DEFINING AN AREA OF FLOW NO MORE THAN 3/4 AND NO LESS THAN 1/4 THE AREA OF FLOW OF THE REMAINDER OF SAID FIRST CONDUIT, SAID RESTRICTION MEANS DEFINING AN INNER AND OUTER SURFACE WITH THE OUTER SURFACE COOPERATING WIHT THE INTERIOR SURFACE OF SAID FIRST CONDUIT TO DEFINE A SPACE, AND INSULATION MEANS FILLING SAID SPACE. 